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Abstract

In this article we describe a cost-effective approach for hybrid laser integration, in which vertical cavity surface emitting lasers (VCSELs) are passively-aligned and flip-chip bonded to a Si photonic integrated circuit (PIC), with a tilt-angle optimized for optical-insertion into standard grating-couplers. A tilt-angle of 10° is achieved by controlling the reflow of the solder ball deposition used for the electrical-contacting and mechanical-bonding of the VCSEL to the PIC. After flip-chip integration, the VCSEL-to-PIC insertion loss is −11.8 dB, indicating an excess coupling penalty of −5.9 dB, compared to Fibre-to-PIC coupling. Finite difference time domain simulations indicate that the penalty arises from the relatively poor match between the VCSEL mode and the grating-coupler.

Figures (9)

Fig. 1 (a) Schematic of a single-mode fibre (SMF) grating-coupled to the test Si-PIC, showing the near-normal angle-of-incidence of approximately 10°, The top-oxide (TOX) layer, the SOI layer, the bottom-oxide layer (BOX) and the substrate (SUB) of the sample. (b) Schematic of the sample used for active-alignment measurements, where the VCSEl is bounted on an AlN sub-mount, bonded to an electrical FLEX connector that provides power to the VCSEL and offers a means of translating and tilting the sample above the grating-coupler. (c) Schematic of the tilted-VCSEL flip-chip bonded above a grating-coupler on a Si-PIC, showing the solder ball deposition (SBD) and wire-bond used to make the n- and p-type electrical-connections to the on-PIC contact-pads and tracks.

Fig. 2 The Fibre-PIC-Fibre transmission (TFPF) spectrum at AOI = 10°, and VCSEL power (PV) spectrum, with a drive-current of ID = 10 mA and a tilt-angle of 10°. The threshold wavelength, FWHM line-width, and suppression of the VCSEL are 1546.15 nm, <0.05 nm, and and 37 dB, respectively. The value of TFPF at the emission wavelength is −11.7 dB.

Fig. 4 (a) The VCSEL power (PV), VCSEL-PIC-Fibre (PVPF), VCSEL-PIC-Fibre transmission (TVPF), and Fibre-PIC-Fibre transmission (TFPF) as a function of drive-current (ID). As expected, for drive-currents higher than approximately twice the threshold, the VCSEL-PIC-Fibre transmission is independent of ID. Given that the average value of TVPF is −16.2 dB and that TFPF = −11.7 dB at the emission wavelength, the VCSEL-PIC insertion-loss (LVP) is −10.4 dB. This corresponds to an excess coupling-penalty of (LEX) of −4.5 dB, compared to the Fibre-PIC insertion-loss of the same grating-coupler. (b) and (c) Schematic of the mode-field diameter (MFD) of the fibre- and VCSEL-mode reaching the grating-coupler on the PIC surface

Fig. 5 3D-FDTD simulations of the Fibre-PIC and VCSEL-PIC insertion losses to the standard grating-coupler used in the experimental measurements, and the reduced VCSEL-PIC insertion-loss for coupling to an optimized large-footprint grating-coupler.

Fig. 6 Alignment tolerance of the VCSEL across (X) and along (Y) the symmetrical axis of the grating-coupler, made using active-alignment VCSEL-PIC-Fibre (PVPF) measurements at ID = 10 mA. The 1dB alignment tolerance is ± 1.6 µm in both directions. The inset shows a plan-view of the corresponding grating-coupler structure studied in the 3D-FDTD simulations.

Fig. 7 (a) A series of SEM images showing solder ball deposition (SBD) deposited on test-structures, to calibrate the height of the SBD-reflow as a function of the contact-pad area. (b) Plot showing the SBD-reflow height as a function of square contact-pad width, and the corresponding tilt-angle of the VCSEL on the PIC.

Fig. 8 (a) Schematic of the relevant area of the Si-PIC, showing the grating-coupler, and waveguide, the solder ball deposition (SBD), and the Au-tracks and bond-pads for contacting the VCSEL. (b) Schematic of the VCSEL, mounted on the flip-chip pick-up tool, showing the bond-pads for electrical-connection and the aperture for laser emission. (c) Combined image of the Si-PIC and VCSEL from the flip-chip bonder, which uses a beam-splitting mirror to simultaneously image both components, to allow for precision alignment.

Fig. 9 (a) Power spectrum (PVP) and (b) LI-curve of a flip-chip bonded and packaged tilted-VCSEL on the Si-PIC. The emission is centered at 1547.15 nm, and has a polarization/side-band suppression of 35 dB. At roll-over, the maximum optical-power injected into the PIC is 138 µW = −8.6 dBm. The slope-efficiency of the injected power is 1.6%.The inset of (a) shows a microscope image of the VCSEL bonded onto the PIC, before the top-side wire-bond was added, and the inset of (b) shows an SEM image of a 10° tilted-VCSEL on a PIC, with false colors to more easily identify the VCSEL (purple), electrical contacts (gold), SBD (blue), and waveguide structures (green).